Two new and independent studies have put Einstein's General Theory of
Relativity to the test like never before. These results, made using NASA's Chandra X-ray Observatory, show Einstein's theory is still the best game in town.

Each team of scientists took advantage of extensive Chandra observations of galaxy clusters, the largest objects in the Universe bound together by gravity. One result undercuts a rival gravity model to General Relativity, while the other shows that Einstein's theory works over a vast range of times and distances across the cosmos.

The first finding significantly weakens a competitor to General Relativity known as "f(R) gravity".

"If General Relativity were the heavyweight boxing champion, this other theory was hoping to be the upstart contender," said Fabian Schmidt of the California Institute of Technology in Pasadena, who led the study. "Our work shows that the chances of its upsetting the champ are very slim."

In recent years, physicists have turned their attention to competing theories to General Relativity as a possible explanation for the accelerated expansion of the universe. Currently, the most popular explanation for the acceleration is the so-called cosmological constant, which can be understood as energy that exists in empty space. This energy is referred to as dark energy to emphasize that it cannot be directly detected.

In the f(R) theory, the cosmic acceleration comes not from an exotic form of energy but from a modification of the gravitational force. The modified force also affects the rate at which small enhancements of matter can grow over the eons to become massive clusters of galaxies, opening up the possibility of a sensitive test of the theory.

Schmidt and colleagues used mass estimates of 49 galaxy clusters in the
local universe from Chandra observations, and compared them with theoretical
model predictions and studies of supernovas, the cosmic microwave background, and the large-scale distribution of galaxies.

They found no evidence that gravity is different from General Relativity on scales larger than 130 million light years. This limit corresponds to a hundred-fold improvement on the bounds of the modified gravitational force's range that can be set without using the cluster data.

"This is the strongest ever constraint set on an alternative to General Relativity on such large distance scales," said Schmidt. "Our results show that we can probe gravity stringently on cosmological scales by using observations of galaxy clusters."

The reason for this dramatic improvement in constraints can be traced to the greatly enhanced gravitational forces acting in clusters as opposed to the universal background expansion of the universe. The cluster-growth technique also promises to be a good probe of other modified gravity scenarios, such as models motivated by higher-dimensional theories and string theory.

A second, independent study also bolsters General Relativity by directly testing it across cosmological distances and times. Up until now, General Relativity had been verified only using experiments from laboratory to Solar System scales, leaving the door open to the possibility that General Relativity breaks down on much larger scales.

To probe this question, a group at Stanford University compared Chandra observations of how rapidly galaxy clusters have grown over time to the predictions of General Relativity. The result is nearly complete agreement between observation and theory.

"Einstein's theory succeeds again, this time in calculating how many massive clusters have formed under gravity's pull over the last five billion years," said David Rapetti of the Kavli Institute for Particle Astrophysics and Cosmology (KIPAC) at Stanford University and SLAC National Accelerator Laboratory, who led the new study. "Excitingly and reassuringly, our results are the most robust consistency test of General Relativity yet carried out on cosmological scales."

Rapetti and his colleagues based their results on a sample of 238 clusters detected across the whole sky by the now-defunct ROSAT X-ray telescope. These data were enhanced by detailed mass measurements for 71 distant clusters using Chandra, and 23 relatively nearby clusters using ROSAT, and combined with studies of supernovas, the cosmic microwave background, the distribution of galaxies and distance estimates to galaxy clusters.

Galaxy clusters are important objects in the quest to understand the Universe as a whole. Because the observations of the masses of galaxy clusters are directly sensitive to the properties of gravity, they provide crucial information. Other techniques such as observations of supernovas or the distribution of galaxies measure cosmic distances, which depend only on the expansion rate of the universe. In contrast, the cluster technique used by Rapetti and his colleagues measure in addition the growth rate of the cosmic structure, as driven by gravity.

"Cosmic acceleration represents a great challenge to our modern understanding of physics," said Rapetti's co-author Adam Mantz of NASA's Goddard Space Flight Center in Maryland. "Measurements of acceleration have highlighted how little we know about gravity at cosmic scales, but we're now starting to push back our ignorance."

The paper by Fabian Schmidt was published in Physics Review D, Volume 80 in October 2009 and is co-authored by Alexey Vikhlinin of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, and Wayne Hu of the University of Chicago, Illinois. The paper by David Rapetti was recently accepted for publication in the Monthly Notices of the Royal Astronomical Society and is co-authored by Mantz, Steve Allen of KIPAC at Stanford and Harald Ebeling of the Institute for Astronomy in Hawaii.

Visitor Comments (15)

Great information. I got lucky and found your site from a random Google search. Fortunately for me, this topic just happens to be something that I've been trying to find more info on for research purposes. Keep up the great work and thanks a lot.

Posted by J D on Monday, 02.21.11 @ 17:38pm

chandra.harvard.edu did it again. Incredible writing.

Posted by Johnie Buck on Monday, 05.31.10 @ 05:43am

Dear Len,
The redshifts of the galaxies in the cluster are used, along with a model for the expansion of the universe as dominated by dark energy and dark matter. The details are complicated, but in general the larger the redshift the larger the distance.
P. Edmonds for CXC

Posted by P. Edmonds on Wednesday, 04.28.10 @ 12:12pm

Dear Ved,
This work is testing the *possibility* that the acceleration in the expansion of the universe is because of the modification of gravity. The crucial point is that so far no evidence has been found requiring changes to General Relativity.
P. Edmonds for CXC

Posted by P. Edmonds on Wednesday, 04.28.10 @ 12:09pm

Dear Georges,
Thanks for your good question. To be more precice, the expansion rate is
not sensitive to the properties of gravity when acting over very large scales. For example, in a model of the universe people can change the way gravity acts over large scales so that it differs from the predictions of General Relativity. This reproduces accelerated expansion without the need for dark energy, but this changes the way galaxy clusters grow. So far such deviations from General Relativity have not been seen.
P. Edmonds for CXC

Posted by P. Edmonds on Wednesday, 04.28.10 @ 12:06pm

Dear John,
This is an interesting idea, but even if some mechanism was able to generate magnetic fields over very large scales, it wouldn't be able to have a significant affect since matter on these scales will be electrically neutral. On these scales gravity dominates over magnetic forces.
P.Edmonds for CXC

and for more information about Chandra visit
http://www.chandra.harvard.edu/about/

CXC

Posted by CXC on Wednesday, 04.28.10 @ 11:17am

Would it be possible to have more information about Chandra? Like a portable transmitter like those who are doing that.

Also more info on aerospace assistive medical technologies.

Thank you

Posted by Diego Thornton on Wednesday, 04.21.10 @ 16:48pm

I wonder if anyone ever thought that maybe this Dark Matter Dark Energy is nothing more than all the magnetic forces in the universe pushing and pulling everything around and being such a large magnetic force would be very difficult to detect since it would be on such an epic scale there is not a technology out yet to prove against this theory.

Posted by John on Saturday, 04.17.10 @ 14:32pm

Very nice articles thanks

Posted by Marcel on Thursday, 04.15.10 @ 13:00pm

Interesting article. But your comment about the expansion rate being not sensitive to the properties of gravity is somewhat puzzling. After all, the expansion rate is none other than the Hubble parameter, which is expressed in terms of the energy density rho divided by the square of the gravitational constant G, in addition to lambda and the curvature parameter k. So how could it be said to be insensitive to the properties of gravity?

Posted by Georges Melki on Thursday, 04.15.10 @ 01:21am

Dear sir
As you mentioned that expansion of the universe is not because of exotic energy but from the modification of gravity force. Is it not so that gravity force is also Energy it is all over in the universe?
If I am wrong please forgive me for the mistake.
thanks and best regards

Posted by Ved Parkash on Thursday, 04.15.10 @ 00:27am

How can one tell the difference between 100 million light years away and 10 or 600 million light years away.

I am constantly befuddled by the distances away and across in light years.

I would love a poster of the latest Abell 3376, if it become available.

Posted by Len Gyson on Wednesday, 04.14.10 @ 22:22pm

Wish my life could continue for another 60 years - just to see what we discover. Keep up the good work and I hope sufficient funds will continue so you can continue. Your studies are worth every penny. My hat is off to each of you. Health, Hope, Happiness Mike

Posted by Mike Giambra on Wednesday, 04.14.10 @ 21:48pm

This article and the conclusions are just wonderful to behold too bad I cannot send an email to Albert Einstein letting him know that he was right all along and the inescapable conclusion that the expansion constant is in fact and not an error of understanding on the part of the old man Congratulations to the teams that have done this fine work

Posted by Neil Fiertel on Wednesday, 04.14.10 @ 20:38pm

Leave Your Comment

Note: Keep comments relevant. Inappropriate or offensive comments will be edited and/or deleted. To avoid appearing to endorse commercial goods or services, posts containing URLs may not be published. This post is moderated during normal business hours (eastern U.S. time); it may be several hours before they are posted. Please keep your comment short (not more than 600 characters).

Emails will not be visible or used in any way, and are not required. Privacy Policy